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Beginner

Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Please forgive my utter lack of BGP knowledge. I am new to the networking world, and even moreso, the BGP world.

 

We are having an issue where one of our ISP routers can ping both google.com and 8.8.8.8, but the other can only hit google.com and not 8.8.8.8.

 

In my troubleshooting I noticed that the working router has a proper next hop to the isp when pinging both name and ip. The non working router's next hop is the working router's ip when attemping 8.8.8.8. When pinging google.com from the non working router, it does have its proper next hop of the isp it is connected to.

 

I further verified that when executing sh ip bgp, that the non working router shows a *>i for 8.0.0.0 to the working router's interface, whereas the working router shows the proper *> for 8.0.0.0 to its isp connection.

 

So as I understand it, the non working router is learning incorrect BGP routes? Not sure what else I should be looking for, so any assistance is appreciated.

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7 REPLIES 7
VIP Advisor

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Hello

To assist you further can you post or explain how your routers are connected to each other, your network and the isps



kind regards
Paul

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It will hopefully assist others with similar issues in the future
Beginner

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Hi Paul. Thanks for the reply. They are directly connected on their g0/2 interfaces via ospf. The router that cannot hit 8.8.8.8 is connected to Century Link, and the one that can is connected to Level 3. 

 

Beginner

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

In your iBGP configuration, do you have "next-hop-self" set on both sides?  

 

Also, Google.com for me resolves to 172.217.6.174 (right now) as opposed to their public DNS of 8.8.8.8 and 8.8.4.4.  

 

I suspect that if you are using RFC1918 between your routers (we do that as well) that your source IP is the interface that is cross-connected across to the working router. 

 

 

Some more information as @paul driver  suggested will also be helpful.

 

--tim

Beginner

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Hi Tim. Thanks for the reply. Yes, next hop self is configured as the other router on each router. On the router that cannot ping 8.8.8.8, it can ping 172.217.6.174. Seems odd that it would learn the router for 8.0.0.0/9 as going through the other router. 

 

You suspected right. When tracerouting to 8.8.8.8 from the router that cannot ping it, it does have the source as the interface that cross connects the two.

 

 

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Hall of Fame Guru

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

 

Just to add your non working router is not necessarily learning incorrect routes unless you want everything to go via the directly connected ISP. 

 

It probably just means that as far as your non working router is concerned the better path to 8.8.8.8 is via the other router, better path being determined by the BGP best path algorithm. 

 

So it may or may not be an issue depending on what exactly you want in terms of traffic paths. 

 

Jon

Beginner

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Hi Jon. Thanks for the reply. Definitely understood as far as that logic is concerned. Then my question is why the router still cannot ping 8.8.8.8. If it goes to the other router, that is fine, but I am not understanding where it is getting hung up that it cannot get through the router that can ping 8.8.8.8.

VIP Advisor

Re: Can ping google.com but not 8.8.8.8 - Noticing some BGP route differences

Hello

Can you post a topology for clarity and if possible a configuration of both routers



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
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